Bounds for `RwLock<T>: Send` are unnecessarily restrictive

[RalfJung]

Currently, RwLock<T>: Send requires T: Send and T: Sync. It would be sufficient to requires T: Send. (Transmitting an RwLock can't move an &T across thread boundaries.)

Is there interest in relaxing these kinds of bounds? Or, maybe, at least document the fact that the bounds could be more relaxed in the source?

[dtolnay]

This bound was relaxed in #45267.